Technical Abstract:
Huanglongbing (HLB) was reported in 2004 in São Paulo, Brazil and Candidatus Liberibacter asiaticus and Ca. L. americanus, were found associated with the disease with prevalence of the second species. The Asian psyllid vector of Ca. L. asiaticus, Diaphorina citri Kuwayama, reached Brazil 60 years ago, is well established in São Paulo orchards, and is probably the main candidate of HLB-agent vector in Brazil. However, due to its recent report in Brazilian orchards, the transmission of Ca. Liberibacter ssp. by D. citri in Brazil is not yet confirmed and no data is available with regard to HLB spatial distribution in Brazil. Assessments of diseased trees by visual symptoms were made in 36 groves from 8 farms in the central citrus region of São Paulo State. 155 HLB spatial maps (varying from 0.14 to 25.99% of disease incidence) were analysed, considering quadrat sizes of 2x2, 4x4, 6x6 and 8x8 trees, by ordinary runs analysis, binomial index of dispersion and binary form of Taylor’s power law. Aggregation among HLB-symptomatic trees was detected by ordinary runs analysis, with clustering existed in both within- and across-rows directions. However the percentage of aggregation within- and across-rows were low. The binomial index of dispersion for various quadrat sizes suggested aggregation of HLB-symptomatic trees for about 40% of the plots. The relationship between log(observed variance) and log(binomial variance) was highly significant for all four quadrat sizes. Estimated parameters of the binary form of Taylor’s power law provided an overall measure of aggregation of HLB-symptomatic trees for all quadrat sizes tested. All power law estimates of b and A, were statistically different from 1, which indicated a general and significant pattern of aggregation of symptomatic plants for all quadrat sizes tested. The degree of aggregation was also positively related to disease incidence. Data from 20 plots ranging in disease incidence were also analyzed by spatial autocorrelation to examine the association among groups of infected trees using the 2x2 quadrat size. In 14 of 20 cases, clusters of HLB-infected trees were found to be associated with secondary clusters whose centers were at distances ranging from 4.2 to 22.1 tree spaces distant, indicating psyllid vector movement resulting in transmission both to nearby trees causing clusters and to trees at considerable distance initiating new foci of infection.